A Tale of Two Oceans, and the Monsoons Tiny Seafloor Shells Could Offer Big Clues to the Forces That Generate Rainfall

A tale of two oceans, and the monsoons Tiny seafloor shells could offer big clues to the forces that generate rainfall By Fern Gibbons lion people from their homes. Over the But we can use the past. One way to un- past 200 years, several droughts have each ravel the complexities of the monsoon sys- very summer, the continent of Asia caused more than 5 million deaths. tem is to study how it has worked in the takes a big breath. This inhalation If we could forecast upcoming mon- past. To do this, we use some very small pullsE moisture-laden air from the Indian soon seasons, we could take steps to avoid shells, preserved in sediments at the bottom Ocean over India and Southeast Asia, caus- death and destruction. But the monsoons of the sea, to answer some very big ques- ing torrential rains known as the monsoons. have defied prediction so far, partly because tions about climate. For as long as there have been people in In- they are generated by complex interactions dia and Southeast Asia, lives have been set among land, air, and two oceans. Predicting A seesawing ocean by the rhythm of the monsoons. the monsoons will become even more diffi- During the summer, the huge landmass Some years, too much rain brings devas- cult as global warming creates a climate that of Asia heats up like a brick in the sun; hot tating floods and mudslides; in other years, we have not experienced in recorded history. air rises over the continent, and cool ocean too little rain causes droughts and famine. We won’t be able to use our current climate air, filled with moisture, flows in to replace In 2007, flooding displaced about 30 mil- as a guide to the future. it. During the winter, the air over the Indi- Linking the HEAVY RAINS Indian Ocean H T U G HEAVY RAINS O and the R D monsoons Indian Ocean Surface temperatures WARMER WATER E in the Indian Ocean CLIN ERMO TH COLDER WATER naturally oscillate, a phenomenon called the Indian Ocean POSITIVE PHASE Dipole. During its positive phase, warmer waters in the western Indian Ocean U G H T O bring heavy rains to R East Africa and India, D and colder waters bring HEAVY RAINS drought to Southeast Asia. In the negative Indian Ocean phase, ocean and monsoonal conditions reverse. WARMER WATER T COLDER WATER HER MOC LINE NEGATIVE PHASE E. Paul Oberlander, WHOI 6 OCEANUS MAGAZINE Vol. 47, No. 1, 2008 www.whoi.edu/oceanus an Ocean is warmer than the air over land; the ocean air rises, drawing cool, dry air from Asia out to sea. The size of the air mass that Asia breathes in depends, in part, on the temperatures of the Pacific and the Indian Oceans. And the temperatures of these oceans are influenced by two large-scale, dynamic patterns of winds and water temperatures that seesaw between two extremes: El Niño- Southern Oscillation (or ENSO) and the Indian Ocean Dipole. During “normal” years in the Pacific Ocean, trade winds blow along the equator from Central America toward Indone- sia, pushing warm surface water toward the western Pacific. In the eastern Pacific, cool water from the deep ocean is pulled up to the surface to replace the water blown to the west. The constant blowing of the trade winds maintains a Pacific Ocean with a tilted surface. So much warm water piles up in the west, the sea surface height there is about 7 inches higher than in the east. Occasionally, the trade winds will weak- en just for a bit. Without the winds hold- ing the mound of warm water in place, it begins to slide back to the east. This is an “El Niño” event. As the pool of warm water from the western Pacific spreads eastward across the Pacific, it creates a cap of warm water in the east, effectively preventing the cool deep water from reaching the surface. During El Niño years, the entire Pacific is warmer than usual. That can take Asia’s breath away during the summer. A hotter Pacific Ocean can effectively compete with the hot Asian landmass, so that no cool, moist air is drawn landward, and ultimately, the monsoons do not fall over India. Back and forth in the Indian Ocean The Indian Ocean has its own seesaw behavior called the Indian Ocean Dipole. During a “positive” Indian Ocean Dipole event, warm waters in the east spread toward the west, somewhat as in an El Niño. The winds that blow along the equator also switch directions and blow toward the west. These westward-blowing equatorial winds enhance monsoon winds and create a more powerful monsoon. Kleindinst,Tom WHOI India’s largest rainfall in the past 150 MIT/WHOI graduate student Fern Gibbons extracts tiny preserved shells of marine life from years occurred during a positive Indian sediments cored from the seafloor. The shells hold chemical clues that reveal ocean conditions Ocean Dipole event. During negative In- in the past and possible changes in the future. WoodS HOLE OCEANOGRAPHIC INSTITUTION 7 climate conditions that extend many thou- How the Pacific influences rainfall in Asia sands of years into the past. El Niño/Southern Oscillation periodically shifts Pacific Ocean temperatures. When Back on land, the cores are sliced open. surface temperatures in the eastern equatorial Pacific are warmer (orange) during We scope out slices of mud with a little spat- an El Niño, there are heavy rains in East Africa and droughts in India, Indonesia, and ula. When the cores are fresh, the mud they Australia. When eastern Pacific waters turn colder (blue) during a La Niña, rainfall contain has a consistency similar to cake patterns flip-flop in Asia, Africa, and Australia. frosting. We rinse the mud in a sieve so that the remaining sample is mostly shells. With the naked eye, the shells look like grains of sand. Under a microscope, they are beauti- ful, shaped in spirals, disks, spheres, tubes, and cones. There are shiny shells and pitted shells and even the occasional pink shell. From the core mud, I pick out two species of foraminifera, G. ruber and P. obliquiloculata, and move them to a separate dish with a very fine paintbrush. G. ruber lives right at the surface of the ocean, while P. obliquiloculata lives deeper, at approximate- ly 300 feet. So measuring the isotopic and chemical composition of both of them is like having two thermometers in the water, one at the surface and one 300 meters deep. If both foraminifera record warm temperatures, that means the layer of warm water on the surface was thick. If G. ruber records warm temperatures and P. obliquiloculata records cold temperatures, that means the warm surface layer was thin. While measuring two species is twice the work, knowing the temperature of both surface and subsurface waters is important if we want to understand how the mechanisms of the climate system interact. Elementary evidence dian Ocean Dipole events, ocean and mon- you saw in the pond water in junior high We analyze the shells’ chemistry in a soonal conditions reverse. school, these amoebas build shells around mass spectrometer, investigating ratios of Predicting the monsoons is so difficult themselves, which they make from calcium carbon, oxygen, calcium, and other elements because ENSO and the Indian Ocean Di- carbonate. The shells contain evidence of that sometimes may take the place of calci- pole seem to take turns influencing the what seawater conditions were like when um in the shell while it was growing. Varia- monsoons. In some years, an El Niño event the foraminifera were living and building tions in these elements can tell us about has a strong influence on the monsoons, their shells. the temperature and salinity of the seawa- and the Indian Ocean Dipole has none. In A foraminifer lives for about a month. ter that existed when the foraminifera were other years, it is vice versa. The reasons for When it dies, it sinks to the seafloor. It alive. They can also tell us about past lev- this trade-off are not entirely clear, but it falls on top of the dead foraminifera from els of freshwater runoff into the ocean from probably has to do with the exact timing, the previous year, and next year’s foramin- rivers; increases indicate times in the past strength, and positioning of the warm water ifera will fall on top of it. Research vessels when monsoonal rainfall had risen. pools during El Niño and Indian Ocean Di- can collect a core of these sediments, us- With my Ph.D. advisor, Delia Oppo, pole events. ing a large tube inserted into the seafloor. I am examining sediments that have been From the core, we can extract and analyze cored from the seafloor in the eastern Indian The past holds clues to the future foraminifera shells to reconstruct ocean Ocean, along the coasts of the islands of Java In our quest to study past climate, our conditions, and we can date the shells to de- and Sumatra. In this location, ocean condi- main clues come from foraminifera. These termine when those conditions were occur- tions are influenced by ENSO, the Indian are amoebas, but unlike the tiny blobs ring. Together, these let us create records of Ocean Dipole, and the monsoons. By exam- 8 OCEANUS MAGAZINE Vol. 47, No. 1, 2008 www.whoi.edu/oceanus ining several cores from this region, we hope to tease out how these three systems interact.

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